Normalize track lengths with gaps before flatten stacks

[markreidvfx]

This PR fixes #1430

I added a _normalize_tracks_lengths function that adds gap items to the end of tracks that are shorter before the flattening happens.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.00000% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 79.76%. Comparing base (013fe04) to head (d5430bd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1703      +/-   ##
==========================================
- Coverage   79.83%   79.76%   -0.08%     
==========================================
  Files         197      197              
  Lines       21814    21846      +32     
  Branches     4362     4380      +18     
==========================================
+ Hits        17416    17425       +9     
- Misses       2233     2246      +13     
- Partials     2165     2175      +10     

Flag	Coverage Δ
py-unittests	79.76% <30.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/opentimelineio/stackAlgorithm.cpp	28.71% <30.00%> (+0.54%)	⬆️

... and 1 file with indirect coverage changes

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[jminor]

Looks good! I left some requests for comments in a few places. Overall it looks correct to me.

[markreidvfx]

I think I addressed your notes. please take a look when you get a chance :)

[jminor]

Looks great! Thanks Mark!